Changing the Ring and pinion

[aristico]

I have 68 Ford F100 that has a 308 gear ratio. When I'm driving down the highway or just in the streets and I let off the gas it feels like I'm hitting the brakes. I've been told that changing the ring and pinion would take care of that so it will cruise better and in the process improve fuel mileage a bit. Is those so or are they talking out of their butts? If it is the case what gear ratio should I be looking at?

Thanks

[mrtleavitt]

A 3.08 gear ratio is a very good highway gear. If it feels like you're hitting the brakes when you let off the gas I'd look into that a bit more. Look for sticking brakes, bad wheel bearings, or something like that. But on the other hand it is not hard getting the gears out of the truck but I always have a reputable shop do the actual gear changing so that they can setup the gears with the proper lash.

[sargentrs]

I just recently replaced my 3.89 gears with 3.00's for the highway cruising speeds at lower rpm's. I agree with mrtleavitt about the sticking brake scenario. The 3.89's I had really dragged it down when you left off the gas, like you described, and the 3.08's should be a lot smoother transition. I'd look for other reasons too. Also, having just done my gears, if you're not familiar with the process take it somewhere to be set. It's a real pain without the proper tools and equipment.

[mybros71]

I agree with the previous two posts. But I think you might be surprised with your ability to replace and set your own ring and pinion. The 9-inch is the easiest I've ever set up, because you can do it standing up instead of laying under the car and you don't need to install and remove the differential to adjust shims.

If you have 3.08:1 gear ratio in your rear end, it means that every time your tires make one revolution, the pinion gear and drive line make approximately 3.08 revolutions. Compare this to a numerically higher gear ratio of 4.88:1 and you can see that the 3.08 allows less mechanical advantage (MA) for the engine and transmission due to difference in moment-arm length. This results in less accelleration for a given horsepower but lower engine and drive train RPM at a given speed. Exactly the same thing happens when you decelerate. The drive train has less MA to slow the vehicle and you get a slower/smoother deceleration based upon engine braking.

If you were to go to a numerically lower gear ratio, I suspect you would run into a fuel economy issue due to a nessecity/desire to add more gas pedal to attain a more pleasing acceleration rate in traffic.

All this being said, if you have an automatic, make sure your transmission is actually shifting to high gear too. I've experienced this before.

[BobbyFord]

I doubt you'd want to go to a taller gear than 3.08 for improved highway mileage. I have 3.00 in my '63 and it's just fine for highway and city driving combined.

[aristico]

Thanks for the advice guys. I'm going to have my mechanic look for the other things you mention might be a problem since most agree a 308 should be good enough for cruising.

[bmcgc]

Im changing my 3.25 to a 2.47.

I do some highway driving and it tops out at about 60mph. Way to slow for Atlanta traffic.

[mybros71]

Im changing my 3.25 to a 2.47.

I do some highway driving and it tops out at about 60mph. Way to slow for Atlanta traffic.

What transmission do you have? What size tires? I'm having dificulty understanding how this could be.

[bmcgc]

Ive got a 3 speed manual and 235-75-15s.

[BobbyFord]

I have 245/60/15 tires and 3.00 gears in my '63 and it will run 90 all day long. Sometimes a little more...

[hazelnut]

Thats about where the speedo needs to be when your getting on the atlanta downtown expressway. Anything below 85 somebodys going to be running over you LOL.

[sargentrs]

Ive got a 3 speed manual and 235-75-15s.

That was exactly what I started with and she had the 3.25 gears. She'd cruise at 80mph and I could still hear the radio. Then upped the tires to 265-75/15's and stopped listening to the radio. Then went to the 3.89's and I stopped trying to hear anything at 70mph.

[bmcgc]

Thats about where the speedo needs to be when your getting on the atlanta downtown expressway. Anything below 85 somebodys going to be running over you LOL.

We call that the Watermellon 500!

[papabug71]

Im changing my 3.25 to a 2.47.

I do some highway driving and it tops out at about 60mph. Way to slow for Atlanta traffic.

Dang. 2:47's are salt flat gears! If all you can get is 60 mph with 3:25's , you have some problems. I've pegged the speedo with 3:25's. Granted, the little 302 was wound up tight at 100, but it'll cruise at 60 or 70 no problem.